This invention relates to the measurement of liquid level within a fluid reservoir and especially to apparatus for such measurement of the type in which a float confined to vertical movement within a tube open to the reservoir incorporates a magnet to activate magnetic sensors such as reed switches in vertically spaced arrangement adjacent the tube.
In such apparatus, there is a tendency for the float to stick at a vertical location within the tube in spite of changing liquid level and give a false reading. This tendency is the sum of several factors, the first of which is the attraction of the magnet to the reed switch itself. A reed switch has a pair of magnetically responsive electrical contacts which potentially form part of a magnetic circuit if a magnetic field is introduced. The magnetic field, which may originate in a magnet moved adjacent the reed switch, magnetizes the electrical contacts to move into engagement and complete an electrical circuit. However, the magnet, being part of the magnetic circuit, is also attracted to the reed switch. This means, first of all, that some energy must be provided to the magnet to move it away from the reed switch to allow the contacts to open.
In addition, however, the float which contains the magnet is biased by this attraction against the inner surface of the tube wall adjacent the reed switches. This creates friction between the float and tube, due to liquid surface tension. This is particularly true for a float having a flat side against the tube; however, it will still be true to some extent with a rounded side, as long as the float must move vertically with a sliding motion against the side of the tube. This frictional force is added to the attraction of the magnet to the closest reed switch and helps deter the float from vertical movement as liquid level changes. The strongest force against vertical float movement is achieved when the magnet is horizontally adjacent and therefore closest to one of the reed switches with the reed switch contacts closed and the magnetic air gap thus minimal.
In addition to being confined to an essentially constant distance from the plane of the reed switches as it moves vertically, the float of such an apparatus must maintain the poles of the magnet in a substantially constant orientation relative to the reed switches so as to produce consistent magnetic characteristics over time and from one reed switch to the next. Thus the float itself must maintain a predictable orientation in spite of the forces produced by vehicle operation on the float itself and on the liquid.